Exemplary embodiments of the invention relate to a fuel cell stack and a fuel cell system having a fuel cell stack.
Fuel cell stacks comprising a multiplicity of individual cells are generally known as a traditional structure for fuel cells. Here, the fuel cell stack is typically formed from a larger number of substantially prismatically shaped individual cells, which are built up to form a stack, or which consist of substantially prismatic individual parts, which are only finally assembled to form the actual individual cell in the fuel cell stack. This structure is generally known, for example in PEM fuel cells, wherein each of the individual cells has a so-called membrane electrode arrangement (MEA) comprising the membrane as well as two gas diffusion layers and electrodes. Here, a so-called flow field is provided on both the anode side and on the cathode side to supply these electrodes with the appropriate educts. The anode flow field has a multiplicity of channels that feed hydrogen to the anode, while the cathode flow field correspondingly has a multiplicity of channels that feed oxygen or air as an oxygen supplier to the cathode.
When a fuel cell or fuel cell stack is operated, electrical energy is generated from hydrogen and oxygen. As well as the electrical energy, product water, which forms at the individual electrodes and has to be discharged via the flow fields, is also shed by the educts used. In doing so, the greater part of the product water is formed on the cathode side, but a certain amount of formed product water also needs to be discharged on the anode side.
In order to guarantee a good discharge of the product water in all operating situations of the fuel cell stack, the structure described in German patent document DE 10 2006 039 105 A1, for example, attempts to improve the discharge of the product water by an inclined orientation of the flow fields of the fuel cell stack. This is possible in principle, wherein the stack has the problem that, when a vehicle, which is fitted with the fuel cell system for example, is in an inclined position, the flow fields very easily revert to a horizontal or nearly horizontal position, as a result of which, in this operating situation, the discharge of water with the assistance of gravity is then correspondingly jeopardized. A comparable embodiment is also disclosed in Japanese patent document JP 2011 009 137 A.
As well as the problem already addressed, it is the case in both embodiments that, in the usual manner, the flow fields have the media inlets on the one side and the media outlets on the opposite side. Stacking the individual cells on top of one another therefore results in a media inlet channel for each of the educts running through the stack in the stacking direction and is typically arranged at the top, while a media outlet stack running through the fuel cell in the stacking direction is ideally arranged at the bottom.
In practice, particularly when deployed in vehicles, these fuel cell stacks are still very difficult to drain, as a result of which channels within the flow fields are occasionally blocked with water, which can lead to an uneven distribution of educt gases. This can have an adverse effect on the performance of the fuel cell stack and can damage, or in the worst case reverse, the polarity of individual cells due to the lower voltage generated thereby in this case.
Exemplary embodiments of the present invention are directed to a fuel cell stack that avoids these recognized disadvantages and that guarantees safe and reliable draining of its flow fields in all operating situations and in all orientations of the fuel cell stack that typically occur when deployed, for example in a vehicle.
In a fuel cell stack according to the invention, the anode flow fields and/or the cathode flow fields have the media inlet on their upward-facing side when used as intended, and are open on their downward-facing side when used as intended. According to the invention, the anode flow fields and/or the cathode flow fields are not provided with an outlet channel for the medium running through the fuel cell stack in the stacking direction on their downward-facing side when used as intended, but are open, that is to say open to a chamber adjacent to the flow field on the appropriate lateral edge. The flow fields that are therefore formed perpendicular to the stacking direction enable the exhaust gases or exhaust air and the product water contained in these gases to escape from each individual flow field directly into the adjacent chamber. As a result, the discharge of product water formed is improved, even with a corresponding inclined position of the fuel cell stack, as the water can escape directly from the appropriate lateral edge of the flow field without first having to flow via a pipe running in the stacking direction.
In principle, it can be sufficient to design either the anode flow fields or the cathode flow fields in the manner described, wherein the cathode flow fields in particular are preferably to be designed in the manner according to the invention due to the larger amount of product water formed on the cathode side.
However, in a particularly favorable and advantageous development of the fuel cell stack according to the invention, both the anode flow fields and the cathode flow fields are open on their lateral edge facing downwards when used as intended. This enables ideal draining of both the anode side and the cathode side of the fuel cell stack.
Further, in a very advantageous embodiment of the fuel cell stack according to the invention, the chamber or chambers facing the open flow fields or open flow field is designed as a liquid separator or opens out into such. This structure of the chamber facing the open flow fields as a liquid separator or as a flow-carrying pipe leading to a liquid separator is particularly efficient, not only to remove the liquid from the flow fields of the fuel cell stack but also to safely and efficiently separate it from the exhaust gases of the fuel cell stack. As a result of the chamber adjacent to the open flow fields which, in cross-section, ideally widens considerably with respect to the cross-section of the flow fields through which liquid can flow, the flow is calmed and liquid water droplets can fall as rain. Ideally, the liquid separator is then arranged at the bottom in the direction of the gravitational force when used as intended and ideally integrated into the fuel cell stack. This results in a simple and compact structure for the fuel cell stack. By integrating the liquid separator into the fuel cell stack, it is heated together with the fuel cell stack and cooled together therewith relatively slowly so that, in the event of condensation, condensate that is formed can also be separated in the liquid separator. Even at temperatures in the environment of the fuel cell stack below freezing, the liquid separator and the liquid-carrying pipes still remain warm for a relatively long time and therefore at temperatures above freezing. This prevents early freezing of the liquid separator and the liquid-carrying pipes. The resulting condensate can therefore still flow out of the region of the fuel cell stack for a long time, thus minimizing the formation of ice in the fuel cell stack.
To further reinforce this effect, in an advantageous development, an integral drainage pipe connects the liquid separator, which is connected to the anode flow fields, to the liquid separator, which is connected to the cathode flow fields. The liquid separators can be connected to one another by means of such a drainage pipe. At the same time, this drainage pipe is also designed integral with the fuel cell stack so that it remains at a higher temperature for a comparatively long time after switching off. As a result of connecting the two liquid separators to one another, one of the liquid separators, ideally the larger liquid separator on the cathode side, is used to collect all the water. This can then be drained by means of a common pipe and an appropriate valve, for example from time to time, depending on the filling level or similar. As a result, only one valve and one control strategy is required as well as any sensors which may be necessary.
In a further very favorable embodiment of the fuel cell stack according to the invention, the stacked individual cells are bounded on at least one side by an end plate, wherein the at least one end plate extends in the stacking direction both next to the stacked individual cells and also parallel to the stacked individual cells. In this particularly favorable embodiment of the fuel cell stack according to the invention, such an end plate of the fuel cell stack can be designed with an approximately L-shaped contour. On one side of the fuel cell stack, it forms its lateral boundary and also extends at least partially parallel to the fuel cell stack. An appropriately high strength of the fuel cell stack is achieved by this means with minimal outlay regarding its support.
According to a particularly advantageous development, the part of the at least one end plate extending parallel to the stacked individual cells encompasses the chamber/chambers adjoining the open sides of the flow fields. The chambers provided for collecting the exhaust gas and/or the exhaust air and the product water contained therein can be integrated into the part of the end plate extending parallel to the stacked individual cells. This results in a particularly simple, compact and effective structure for the fuel cell stack.
At the same time, according an advantageous embodiment of this aspect the part of the at least one end plate extending parallel to the stacked individual cells has at least one liquid separator arranged at the lowest point when used as intended. The chambers can therefore be linked to a liquid separator or be designed as such, this being arranged within the part of the end plate extending parallel to the stacked individual cells at the lowest point when used as intended. As a result, the water can ideally be collected in this area using the gravitational force.
Further, in another very favorable embodiment one of the end plates, in particular in the top part of the end plate when used as intended, incorporates a gas jet pump for recirculating anode exhaust gas. Such a gas jet pump as a recirculation pumping device for recirculating anode exhaust gas can advantageously be integrated into the end plate.
In a particularly favorable development hereof, the gas jet pump is arranged in the end plate with the part extending parallel to the stacked individual cells, wherein an integrated recirculation pipe in the end plate runs from the region in which the anode flow fields are open to the gas jet pump, and wherein the recirculation pipe has a direction component in each case running in the direction of the gravitational force when used as intended and is connected to the liquid separator at its lowest point. Integrating the gas jet pump into the end plate incorporating the part running parallel to the stacked individual cells enables a recirculation pipe to be integrated into the end plate, which pipe first runs upwards along the stack at an angle to the stacking direction and then perpendicular to the stacking direction in the part of the end plate lying adjacent to the stacked individual cells. Together with the integral gas jet pump, the whole anode recirculation system can therefore be integrated into the end plate. As, when used as intended, the recirculation pipe always has a direction component running in the direction of the gravitational force, that is to say initially at an angle upwards and then, for example, vertically upwards, the remaining water carried along with the recirculating gas flow can be slowed down by the gravitational force so that it runs back along the walls of the recirculation pipe into the liquid separator connected to the recirculation pipe at the lowest point thereof. In this way, in addition to the liquid separation in the chamber and/or liquid separator arranged in the end plate, separation can also be implemented in the region of the recirculation pipe. This is particularly advantageous, as a comparatively large amount of water compared with the volume flow is present particularly in the recirculating anode exhaust gas, and as gas channels can be very easily blocked due to this water in the anode flow fields, particularly in partial load operation of the fuel cell when the freshly supplied fuel flow is comparatively small. Particularly in these situations, water separation that is as ideal as possible in the smallest possible space, which can be achieved by the end plate structure according to the invention, is of particular advantage.
The fuel cell stack according to the invention in one of the described embodiments is used in the fuel cell system according to the invention. The flow fields that are open at the bottom when used as intended already enable very good drainage of the fuel cell stack to be achieved regardless of the orientation of a vehicle which is, for example, fitted with the fuel cell system, as, for example, with a vertical alignment of the flow fields, slight deviations from the vertical direction lead to hardly any impairment of the effect of gravity on the liquid drops. In the fuel cell system according to the invention, this structure is now further improved in that, when used as intended with regard to at least one of the spatial directions perpendicular to the stacking direction, the fuel cell stack is aligned at an angle of more than 10°, preferably approximately 17° to the horizontal. In the fuel cell system according to the invention, the fuel cell stack can therefore be arranged on one edge, for example, or also, in the case of an angle in both spatial directions perpendicular to the stacking direction, on one corner, for example, when, symbolically, a substantially prismatic stack is assumed. Such an arrangement can additionally assist the drainage of water with the help of gravity, wherein, as a result of an angle of more than 10°, in particular approximately 17°, the test conditions and empirical values relating to inclines, inclined orientation of the vehicle etc., which are usually applied in the case of vehicles, are addressed. Even with an inclined orientation of the vehicle of 17°, which is typically assumed to be the maximum, drainage can still be realized with the help of gravity by an appropriate tilting of the fuel cell stack, as the fuel cell stack in turn is correspondingly tilted in the fuel cell system so that an inclined position of the vehicle is at least compensated for by the tilt.
In a favorable embodiment of the fuel cell system according to the invention the fuel cell stack has an end plate with integral gas jet pump. According to the invention the gas jet pump is arranged in the top end plate in the stacking direction when the fuel cell stack is used as intended. This arrangement of the gas jet pump in the top end plate in the stacking direction guarantees that liquid from the gas jet pump drains in the direction of the stack or the recirculation pipe, so that the risk of the gas jet pump being impaired in its operation by liquid water or becoming blocked can be prevented.
Advantageous embodiments and developments of the fuel cell stack according to the invention and of a fuel cell system incorporating the fuel cell stack according to the invention are clear from the exemplary embodiment described in more detail below with reference to the figures.